I used to hunch over a lot too.
I changed to a different kind of sports bra that sort of forces my shoulders back. I also do a lot of stretching in the saddle.
I’ll lift my arms up in the air to get myself seated properly and this will sometimes help me to.
But the biggest thing that worked for me was just constantly telling myself…sit up straight, heels down, sit up, heels down, sit up, heels down, sit up.
now I don’t have to think about it.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."